The International Telecommunications Union Telecommunications Standardization Sector issued recommendation V.34 entitled "A Modem Operating at Data Signaling Rates of Up to 33,600 bit/s For Use on the General Switched Telephone Network and on Leased Point-to-Point Wire Telephone-Type Circuits." The V.34 recommendation was originally adopted in 1994, and revised in early 1996. That standard is hereby incorporated by reference in its entirety.
The V.34 recommendation is the first modem standard which is able to fully adapt to channel characteristics. Among others, V.34 will adjust to carrier frequency, the symbol rate and the transmitted power to optimally fit into the communications channel. V.34 uses a method of handshaking grouped into four phases:
Phase 1: Network interaction(V.8 handshake) PA1 Phase 2: Line probing PA1 Phase 3: An equalizer and echo canceler training PA1 Phase 4: Final training.
The equalizer and echo canceler training is sometimes referred to as half duplex training and the final training is sometimes referred to as full duplex training.
In phase 1, call and answer modems exchange initial handshaking signal using V.8, which was introduced together with V.34 to define the information sequences exchanged. V.8 uses the 300-bps standard V.21 to exchange information with a remote modem on available modulation modes and supported standards and what kind of error correction is to be used, among others.
During the line probing phase, the modem sends regularly spaced tones (every 150 hertz, covering the whole channel bandwidth). Due to the limited bandwidth and channel distortion, each and every one of the probing tones is altered in a different way. This allows one to determine the useable bandwidth and the optimal carrier frequency (which should be centered in the used bandwidth). Each of these tones is sent twice, at two volume levels, which allows one to estimate the influence of nonlinear distortion on the line. What's more, since the tones are at distinct frequencies, it even allows one to estimate the noise present on the line. These estimates can be used to determine the optimal transmit level. At the end of Phase II, two modems will exchange the results in the "INFO 1" sequence. Among the exchanged information are transmit power, carrier frequency, symbol rate and maximum data rate.
During equalizer and echo canceler training, the receiver equalizer starts to adapt to the channel characteristics and the echo canceler starts to adapt to the echo path characteristics. The echo canceler is a device (actually a digital filter) that gets rid of echo effects on analog phone lines and is important for the full-duplex modems to work efficiently. Without an echo canceler, a modem would hear the echoes of its own transmitted signal as well as the remote signal transmitted by the remote modem, and likely become confused. The equalizer compensates the channel distortion, and thus removes the inter-symbol interference in the received signal to help the correct reception of the remotely transmitted data signal. Such an equalizer is typically a linear digital adaptive filter whose transfer function should be close to the inverse of that of the transmission channel. As a result, the noise at the input to the equalizer, which is typically assumed to be white, will be enhanced by the equalizer, especially when the channel's amplitude distortion is relatively severe. To avoid this problem, V.34 includes an optional technique called precoding. The receiver needs to determine the values for coefficients of a filter based on the noise characteristics at the equalizer output, which is put at the output of the equalizer to whiten and thus reduce the colored noise at the equalizer output, helping the decoder to make correct decisions. The coefficients for the receiver for the noise whitening filter are also sent to the remote modem's transmitter, and used there in a device called a precoder.
During the final training phase, the echo canceller and equalizer coefficients are further refined. The precoding coefficients can be calculated after the echo canceller and equalizer coefficients are stabilized. Then two modems exchange modem parameter sequence MP, which includes modulation and encoding parameters like constellation size, type of trellis encoder, nonlinear encoding and precoding coefficients. A plurality of data rates, symbol rates, and various signal processing techniques permit an optimal fit in almost every channel.
Although the V.34 recommendation specifies the transmit end, the receive end is mostly not specified. Thus, implementers are left to design a receiver which will handle the specified transmitter characteristics as best they can.